Place caster sugar and water in a large saucepan over low heat and stir until the sugar is dissolved . Add the rhubarb ,liqueur ,orange juice and rind and increase heat to medium . cook 5-8mins or until rhubarb is just tender . Allow to cool then strain ,reserving the liquid . set aside . place the mascarpone,cream ,vanilla and icing sugar in a bowl and whisk until thickened . Dip half the sponge fingers into the reserved rhubarb liquid and use them to line the base of a 3.5-litre capacity glass bowl . top with half the rhubarb and spoon over half the mascarpone mixture . Repeat the layers using the remaining biscuits ,rhubarb and mascarpone . Top with cream and dust with cocoa to serve .